1. Field of the Invention
The present invention relates to a pressure abnormality detecting device for detecting abnormality of an injection pressure in an injection molding machine.
2. Description of the Related Art
In an injection molding machine, an injection process is first performed in which an injection screw is advanced to inject molten resin inside an injection cylinder into a clamped mold, thereafter followed by such processes as dwelling, cooling, metering, mold opening, unloading parts, mold closing, and mold clamping. In the injection process, molten resin can choke a mold or a nozzle portion fitted at the end of an injection cylinder by the influence of resin temperature and others, thereby increasing an injection pressure. An excessive increase in pressure will result in damage to a mold, injection cylinder, and nozzle, so that an automatic control has been performed by detecting an abnormal increase in pressure using some sort of means to stop an injection operation.
For example, some methods are known in which a sensor is provided to detect and monitor the injection pressure and when a detected injection pressure exceeds a predetermined upper limit, an injection operation is stopped (refer to Japanese Patent Application Laid-Open No. 62-44416, Japanese Utility Model Application Laid-Open No. 5-58250, and Japanese Patent Application Laid-Open No. 2002-254485).
Even if the injection operation is stopped, a slight advance of the injection screw during deceleration (inertial flow) can not be avoided due to the inertia of a driving system for the injection screw, leading to an abnormal increase in a resin pressure, which may break a mold, injection cylinder, and nozzle. The following method has been known, with the above in view, in which first an increment in pressure during an inertial flow at an injection speed of maximum to zero is obtained, and secondly an increment in pressure during the inertial flow at the current moment is obtained from the increment in pressure during the inertial flow at the maximum injection speed, the current speed of the injection screw, and the maximum injection speed, and then if the sum of the current injection pressure and the increment in pressure during the inertial flow at the current moment exceeds a predetermined limit pressure, the injection screw is stopped (refer to Japanese Patent Application Laid-Open No. 2003-300236).
A control method disclosed in Japanese Patent Application Laid-Open No. 7-1522 does not detect an abnormal increase in the injection pressure, but an injection pressure is predicted and the process is changed from injection to dwelling with consideration for response characteristic (delay time) of an injection actuator. In the control method, a time required to reach the maximum injection pressure is first determined from the maximum injection pressure (set value) for changing the injection process to the dwelling process and the current pressure, and secondly a deceleration time is determined from a predetermined deceleration characteristic and a set injection speed. When the determined deceleration time exceeds the time required to reach the maximum injection pressure, a deceleration pattern for shifting to the dwelling process is generated.
In the foregoing methods disclosed in the publications of Japanese Patent Application Laid-Open No. 62-44416, Japanese Utility Model Application Laid-Open No. 5-58250, and Japanese Patent Application Laid-Open No. 2002-254485, an injection pressure is detected, and if a detected injection pressure exceeds a set upper limit, the injection operation is stopped. However, when the injection pressure has been detected with this method, the injection pressure had already exceeded the set upper limit. For this reason, it is necessary to provide the set upper limit some margin. An excessive margin may stop the injection operation detected as an abnormal injection in spite of a normal injection operation, on the other hand, a too small margin may increase the injection pressure to exceed the set upper limit due to the influence of inertia and others even if the injection operation is stopped, resulting in damage to a mold and the like.
Then, as described in the publication of the above Japanese Patent Application Laid-Open No. 2003-300236, it is assumed that the injection pressure is predicted, and the injection operation is stopped according to the prediction. This method however needs determining an increment in the injection pressure from after the injection operation has been stopped at the maximum injection speed till the speed is reduced to zero (i.e., an increment in the pressure during the inertial flow at the maximum injection speed). When the injection operation is stopped with resin residing in the injection screw, increase in pressure varies with the deceleration distance required until speed corresponding to the current injection speed is reduced to zero and types of resins. Therefore, an increment in the injection pressure from after the injection operation has been stopped at the maximum injection speed till the speed is reduced to zero varies with types of resins, which causes a problem in that the increment needs setting for each of resins to be used.
As a method of predicting the injection pressure, the publication of Japanese Patent Application Laid-Open No. 7-1522 describes a method of predicting a time reaching a pressure at which an injection process is to be changed to a dwelling process. This method controls the time of starting deceleration by predicting a time reaching a pressure at which the injection process is to be changed to the dwelling process. When resin chokes a mold and/or nozzle portion, increase in pressure will be proportional to the amount of movement of an injection screw, not proportional to time. Therefore it is difficult to precisely predict change in pressure due to the choke of resin in a mold and nozzle portion by time.